JP2713932B2 - Image processing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image processing method for correcting input image data. [Conventional example] In a color image input device such as a video camera, R,
In many cases, light transmitted through a primary color filter such as G or B is detected by a light receiving element to output three types of independent color signals. When outputting to an image output device such as a color printer using these color signals, it is necessary to convert the color signals in order to obtain matching according to the color characteristics of both the input device and the output device. For example, FIG. 6-a shows an example of a color signal conversion algorithm when used as a stand-alone type in a digital cutter copier. The 8-bit red R, green G, and blue B signals input from the light receiving element in the color original input device are logarithmically converted by a table converter, and converted into 8-bit density signals cyan C, magenta M, and yellow Y, respectively. In order to match the input / output device, 8-bit C ', M', and Y 'signals are output by the second stage matrix conversion.
The amounts of cyan, magenta, and yellow ink are determined according to the values of the C ', M', and Y 'signals. In this color conversion system, the color characteristics of the input device and the output device are directly linked by the second-stage matrix conversion. In the above example, the color image was expressed with cyan, magenta and yellow inks, but as shown in FIG. 6-b, under color removal (UCR)
By performing black and black extraction, it is also possible to represent four colors of cyan, magenta, yellow, and black. The undercolor removal and black extraction will be described in brief. The smallest value of the density signals C, M, and Y is defined as the black ink amount (Bk), and the cyan, magenta, and yellow signals are represented by C, M, and Y, respectively. That is to say, the values C ′, M ′, and Y ′ are obtained by subtracting Bk from Y. The finally output ink amounts C ″, M ″, and Y ″ are determined by performing masking processing in consideration of unnecessary absorption of ink and the like, thereby obtaining C ′, M ′, and Y ′.
The signal is converted and output. Although the above example is a method in which matching between the reading system and the output system is directly performed, an image is often output in response to an input signal of a chromaticity value in a standard color space such as a uniform color space. For example, an example corresponding to the case of the L ^* a ^* b ^* space will be described below. First, the relationship between the XYZ chromaticity value and the L ^* a ^* b ^* chromaticity value is shown. Becomes Here, X ₀ , Y ₀ , and Z ₀ are X, Y, and Z chromaticity values of the reference white. In the conventional example of the stand-alone type, R, G, B signals were subjected to logarithmic conversion, but here, X, Y, Z similar to RGB values
Values are multiplied by 1/3 power, and L ^* , a ^* , and b ^* values are obtained from their linear transformations. The logarithmic conversion and the 1/3 power conversion become almost the same conversion types if an inversion is added. By performing the masking process by (4 × 4) matrix containing a consideration of constant term of ^{this, L *, a *, b} * * ink amount from the value C, M, can be obtained Y value. Here, the constant term is added for inversion. FIG. 6-c shows these relationships. Further, when under color removal and black extraction are performed to express with four color inks of cyan, magenta, yellow and black, first, as shown in FIG. 6-d, the spectral distribution as shown in FIG. After performing masking processing of a (4 × 4) matrix corresponding to the ideal ink of cyan, magenta, and yellow having, the undercolor removal is performed, and finally (3 × 4) for the cyan, magenta, and yellow inks actually used. 3) A method of performing a masking process or the like can be considered. [Problems to be Solved by the Invention] In the conventional example described above, the input signal values R, G, B or L ^* , a
^In the conversion from ^* and b ^* to the amount of ink, more and more king processing by matrix conversion is performed, but this matrix conversion simply performs processing considering unnecessary absorption of ink, and is based on the color expression area of the output system. No corrective action has been taken. Therefore, when the input image data is outside the color expression area of the output system, there is a problem that the original image cannot be faithfully reproduced. The present invention can reproduce the input image more faithfully by correcting based on the color expression area of the color image output means,
It is an object of the present invention to provide a practical image processing method that is well-balanced in accuracy and configuration. [Means and Actions for Solving the Problems] In order to achieve the above object, the present invention is to input color image data indicating a color original, and based on a distance in a uniform color space, the color image data An image processing method for generating a plurality of density data including black data according to the color image output means, wherein the correction processing is performed using one of the color materials of the color image output means.
The color image data is converted into a color expression area set based on a color that can be reproduced by a combination of a secondary color, a secondary color, and the primary color, the secondary color, and a black color material. Example First, the chromaticity value distribution of ink in a uniform color space will be outlined. Here, an L ^* a ^* b ^* space is used as a uniform color space for convenience. FIG. 1-a shows a chromaticity value of a mixed color patch obtained by printing one or two color inks on white paper among typical three color inks of cyan, magenta and yellow, which is obliquely shaded on an a ^* b ^* plane.
Shown in The half-line extending from the origin in the directions indicated by C, M, and Y indicates the direction of the chromaticity value when the ink amount is increased by using only one color among cyan, magenta, and yellow. It can be seen that the chromaticity change of each ink shown is along this half line within the color difference of 2-3. Here, the ink amounts are selected so that the discrete dot intervals are substantially equal to the color difference, and four points are shown in the figure. Next, chromaticity values are shown for C, M, and Y in FIG.
A point other than the axis. Here, the semi-straight line extending from the origin in the directions indicated by R, G, and B is the red, green,
The average hue of blue is shown, and the deviation between this half line and the actual chromaticity value is also within 2 to 3. Looking further at the quadrangular area surrounded by the dotted line,
The points on the line segment indicated by are arranged at approximately equal intervals of color difference,
The points in the area are also substantially distributed in the vicinity of the intersections of the lines connecting the same points as shown by, for example, a one-dot chain line among the points on the four line segments of,. Is indicated by a black circle). Although the above-described region has two sides of the magenta axis and the yellow axis, the same can be said for the other two combinations. In addition, these quadrangles have shapes substantially similar to parallelograms. From these facts, when expressing a color in a certain hue direction by using one or two of the three inks, it is only necessary to keep the ink amount ratio constant regardless of the saturation value. Recognize. Next, the brightness (L ^* ) and saturation when black ink is added to a combination of three color inks giving a certain hue direction FIG. 1-b shows the change of the above. The change in the hue direction is not shown because it is a minute change of 1 to 2 in color difference. The point on the achromatic axis in FIG. 1-b shows the value when only black ink is used, and the point on the axis shows the value when only chromatic ink is used. The amount has been adjusted. Opposite points on the axis are also distributed at substantially equal intervals. A point in the quadrilateral region is located on the vicinity of the intersection of the lines connecting the same points with the same amount of ink as shown by, for example, a one-dot chain line among the points on the four line segments of,,. Almost distributed (shown here by black circles). These relationships are the same for any hue direction. To summarize, the chromaticity distribution of the color expressed by two of the three chromatic inks of yellow, magenta, and cyan and the one achromatic ink of black with the gradation characteristics adjusted at equal color difference intervals. The hue direction does not change if the ratio of the chromatic ink amounts is fixed. The chromaticity values when no achromatic ink is added are C, M, Y, R, G, B
By giving the direction of the axis, the axis, and the chromaticity value (normal ink amount maximum) of the point with the highest saturation that can be expressed by the ink, all can be determined by the internal division method. Further, when the achromatic ink is added in addition to the chromatic ink corresponding to having a distribution on a certain hue axis, the chromaticity value is the lowest lightness point (maximum ink amount) and the chromaticity value when only the achromatic ink is used. If the chromaticity value of the maximum added point is given while maintaining the hue direction, the ink amount of the chromatic ink can all be determined by the internal division method. It can be said that. Accordingly, the present invention is to easily grasp a color expression area of an output system by using a chromaticity value corresponding to a maximum value of an ink amount and a chromaticity value obtained by mixing a plurality of inks as a parameter, that is, a representative value. Can be. Further, since a color space compression process based on the color expression area can be performed,
An image input in the input system can be reproduced more faithfully in the output system. For converting a color signal expressed on a uniform color space or a color signal that can uniquely correspond to a chromaticity value on a uniform color space into a signal uniquely corresponding to each ink amount or ink amount; As the color conversion parameters, the chromaticity value of the single ink color or the multi-ink color expressed on the image carrier by the ink amount corresponding to the maximum density value of each ink in the uniform color space and the image carrier itself Is used. Embodiment 1 An embodiment in which four types of chromatic inks of three colors of cyan, magenta, and yellow and a single black ink are used will be described. FIG. 2 shows the flow of the color conversion. The details of each stage will be described below. First, the first stage is the determination of the hue direction. FIG. 3 shows a hexagon corresponding to the outermost shell among the points shown in FIG. 1-a, and six vertices correspond to the highest concentrations of C, M, Y, R, G and B. Chromaticity value of the color to be reproduced. Now, assuming that a point is a color that is actually desired to be expressed, a point at which a half line passing through the origin and a hexagon intersects with each other. Since the point belongs to a quadrilateral between the magenta axis and the yellow axis, this hue is represented by a combination of magenta and yellow ink. Since the point is in the direction of the magenta axis when viewed from the red axis, the amount of magenta ink is larger than the amount of yellow ink. When the amount of magenta ink is set to 1, the amount of yellow ink is represented by a line segment. Is given by the ratio to the line segment ▲ ▼. In the second stage, the chromaticity distribution in the hue direction determined in the first stage is determined. FIG. 4 shows the chromaticity values for magenta (M) and red (R) and the chromaticity values M 'and R' when the maximum amount of black ink is added to the chromatic inks corresponding to M and R, respectively. ^* a ^* b ^* space. 2 lines ▲
Assuming that the points where ▼ and ▲ ▼ intersect with the half-plane representing the hue direction obtained in the first stage, the chromaticity value of, is calculated from the chromaticity values of M, M ', R, R' by the internal division method. Easy to find. FIG. 5 shows that in the hue direction obtained in the first stage,
(), And the chromaticity of the image carrier and the chromaticity value when only black ink is given to the maximum. Quadrilateral Indicates an area that can be expressed within this hue. In the third stage, the amount of ink is determined. The internal division ratio of the above quadrilateral is easily determined. If the point of intersection is, the amount of chromatic ink is Multiplied by the ink amount ratio obtained earlier, the amount of achromatic ink is Determined by The color conversion method has been described in three stages, but some examples of actually configuring a digital circuit will be described. FIG. 1 shows a method in which software for executing the above-described sequence is installed in a computer having a CPU, a memory, and the like, and internal division calculations and the like are performed in real time. In this case, there are several ways to select the parameters to be stored in the memory. In the case of the minimum number, only the chromaticity value of the pure color or the mixed color of each ink corresponding to the maximum density value is used. On the other hand, as a parameter,
The values of the vertices of the quadrilateral in each hue half-plane can also be added. The first method described above has a limitation on the calculation speed because the internal division calculation and the like are performed in real time. In such a case, as another method, it is also possible to develop all the calculation results in a memory as a reference table and take out the output value from the memory with respect to the input signal value. As the memory, a ROM created in advance, a RAM storing a value calculated via the CPU when the power to the apparatus is turned on, or the like can be used. The parameters required in this embodiment are the chromaticity values of C, M, Y, R, G, and B corresponding to the maximum density value of the chromaticity value of the image carrier, and Bk (black) of the maximum density value. Are the respective chromaticity values when. When the chromaticity values of the 14 colors are given as color conversion parameters, the amount of ink to be output is determined for the input chromaticity value. As an application example of this embodiment, when the chromaticity value of the color patch output from the image output device is compared with the input signal value using the chromaticity values of 125 standard color patches as an input signal, an average color difference of about 3 to 4 is found. Was. On the other hand, when the conventional method described with reference to FIG. 6-d is applied, an average color difference of about 5 to 6 is observed. In this way, there is no significant difference between the two methods when the average color difference is used as a guide,
In the present embodiment, since the correlation between each ink amount and the chromaticity value in the uniform color space is directly used for the conversion method, a clearer method can be realized as a method of using black toner instead of removing undercolor and extracting black. In addition, there is a great advantage in that the range of color processing, such as processing of colors outside the color expression region of the ink and slightly rotating the hue direction around the achromatic color axis, can be easily expanded. For example, as a processing example of a color signal outside the expression region, it is possible to easily realize, for example, maintaining a hue direction and projecting a color having the least color difference. [Other Embodiments] In the first embodiment, the L ^* a ^* b ^* space is used as the uniform color space. However, even when the L ^* u ^* v ^* space is used, almost the same handling is possible. is there. However, in this case, since the separability in the hue and saturation directions is slightly poor, the color difference is slightly large.
As a result of outputting the chromaticity values of the 125 standard color patches, the average color difference measured in the L ^* a ^* b ^* space was about 5 to 6. In the embodiments described above, the color carrier is expressed by the word ink, but this is not limited to liquid ink, and solid ink on a ribbon used for a thermal transfer printer, powder toner used for electrophotography, etc. It is a general term for various things, and corresponds to coloring materials in a broad sense. In the embodiment, only one kind of the same hue is used for both the chromatic ink and the achromatic ink. The present invention is also applicable to a case where tone expression is performed. [Effects] As described above, according to the present invention, the input image can be reproduced more faithfully by correcting based on the color expression area of the color image output means, and the accuracy and configuration are well-balanced and practical. An image processing method can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1-a shows the color in L ^* a ^* b ^* space of a mixed color patch in which one or two inks of three colors of cyan, magenta and yellow are printed on white paper. the degree value is projected to a ^* b ^* plane figure, for the combination of chromatic inks giving the hue direction the first FIG -b there showed the color representation range in the case of adding the achromatic ink drawing, the 2 is a flowchart of a color conversion method according to the first embodiment of the present invention, FIG. 3 is a diagram for explaining a method for obtaining a mixture ratio of chromatic color ink, and FIG. 4 is a method for obtaining a color expression range in a certain hue direction. FIG. 5 is a diagram showing a method of obtaining each ink amount, and FIG. 6-a is a diagram showing a conventional algorithm of a color conversion method in a stand-alone type color copying machine using only chromatic color inks. Fig. 6-b shows an achromatic color image obtained by performing under color removal in addition to the method a). Shows a conventional example in which click also added, Figure 6 -c standard input signal ^{(L *, a *, b} *) shows the algorithm of the conventional example of determining the chromatic color ink amount with respect to FIG. FIG. 6-d shows a conventional example in which undercolor removal and achromatic ink are added in addition to the method c, and FIG. 7 shows the spectral reflectance of ideal ink.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04N 1/46 B41J 3/04 101A (72) Inventor Akihito Hosaka 3-30 Shimomaruko, Ota-ku, Tokyo 2 Canon Inc. (72) Inventor Kenichi Matsumoto 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-61-288690 (JP, A) JP-A-61 −288662 (JP, A)

Claims (1)

(57) [Claims] Color image data representing a color original is input, a correction process is performed on the color image data based on a distance in a uniform color space, and a plurality of density data including black data corresponding to a color image output unit is generated. In the image processing method, the correction processing is performed based on a primary color, a secondary color, and a color that can be reproduced by a combination of the primary color, the secondary color, and the black color material of the color material of the color image output unit. An image processing method, wherein the color image data is converted into a set color expression area.